A variety of procedures are currently being employed to accomplish removal of cataracts and other material from within the eye. One of those procedures is often referred to as an irrigation-aspiration method. It involves breaking, tearing or otherwise dividing the material to be removed into small fragments and aspirating those fragments from the eye into a fluid flow line by which they are carried away. Fluid pressure levels in the apparatus by which that procedure is practiced are dictated by the need to preserve pressure balance within the eye. More particularly, in the case of cataract removal, it is important to prevent or minimize collapse of the cornea into the cavity formed by removal of lens material. Difficulty in preventing such collapse has tended to discourage some surgeons from the use of the irrigation-aspiration method notwithstanding that it's successful use can result in substantially less discomfort to patients in the post operative period.
In the method, an incision is made on or about the corneal margin to gain access to the lens. A tool in the form of a probe which houses the outlet end of a fluid supply line and the inlet end of an aspiration line is inserted into the opening under the cornea. Some means is provided for dividing the lens material into small fragments. The means for dividing eye material may comprise no more than a cutting edge at or near the flow tube openings. More commonly an ultrasonically driven tool is included to aid in division of the material to be removed. The task is to separate that material and divide it into small pieces, aspirate that material from the eye and to replace the material so removed with water, actually a balanced, salt in water solution. The opening in which the probe is inserted is small, 6 millimeters or less, and the opening tends to close so that a substantially closed cavity is formed as lens material is removed. The water is disposed under the cornea and serves to prevent corneal collapse into the cavity. It is the water in that cavity that is used to aspirate and carry away lens material. Accordingly the flow of water to the lens cavity must equal the flow of water being removed by aspiration augmented by enough water to account for increase in lens cavity size. That flow ratio must be maintained while maintaining enough water in the lens cavity to prevent the cornea's collapse. The inner surface of the cornea is covered by a layer of irreplaceable endothelium cells. Collapse of the cornea would bring those cells into destructive contact with the removal tool. In the case of a sonically activated removal tool, collapse of the cornea into contact with the tool could result in puncture of the cornea. To accomplish the pressure balance to prevent those catastrophes requires precise control of supply water pressure and aspiration pressure. In practice, positive supply pressure is achieved by elevating the supply water container. Aspiration pressure is negative whereby the pressure at the lens cavity is close to atmospheric pressure.
Two primary factors tend to upset the desired pressure level at the eye. Negative pressure in the aspiration circuit is developed by a pump in most practical systems. Peristatic pumps are usually used but whatever the pump form, small cyclic variations in negative pressure occur and tend to make the cornea oscillate over the cavity being formed by the lens. A greater and more troublesome pressure variation occurs when an occlusion or partial occlusion of the aspiration opening is overcome. Occlusion occurs when a piece of eye material too large to pass through the aspiration inlet is drawn to it. Pressure in the aspiration line is forced more negative by the aspiration pump until the blocking material is divided or drawn into the inlet and the occlusion is cleared. When that occurs, the negative aspiration pressure, now greater in absolute value than the supply pressure, evacuates the lens cavity and collapses the cornea. In practice the supply water includes entrained air and there may be bubbles in the supply. The mass of the water and the compliance of the air line, coupled with the reduced flow resistance as the occlusion is overcome, act as an under damped oscillatory system in which the cornea may be vibrated violently as a function of the size of the cavity.